The invention relates to the field of tunable add/drop filters using side-coupled resonant tunneling.
Add/drop filters play a vital role in wavelength division multiplexed (WDM) lightwave communications systems. They allow different wavelengths to be added or removed from an optical transmission line. Add/drop filters come in one of two classes: static or dynamic. Static devices are designed to operate at a given fixed wavelength predetermined during the fabrication process. Dynamic devices are designed to be switched on and off to add or remove selected wavelengths from a transmission line and, in some cases, are also designed to be tunable, i.e., single devices can be made to add or remove any one of a number of different wavelengths.
Examples of dynamic filters include acousto-optic tunable filters (AOTFs), polymer-based thermo-optic switches, and micro electromechanical systems (MEMS). These filters suffer from a variety of physical limitations such as high crosstalk, frequency insensitivity, and overall large size. This latter feature makes some of the filters unsuitable for very large scale integration (VLSI). In fact, a single AOTF can be as large as one square inch.
Recently, an optical switch based on side-coupled resonant tunneling suitable for VLSI has been designed and described in U.S. Pat. No. 6,101,300 entitled xe2x80x9cHIGH EFFICIENCY CHANNEL DROP FILTER WITH ABSORPTIONxe2x80x9d, of common assignee. The switch is activated by changing the internal decaying rate of a resonant coupling element located between two adjacent waveguides. While being very small in size, the switch was designed mostly to add or drop a specific wavelength, determined during fabrication, with limited post-fabrication tunability, i.e., the user could select the wavelength to be added or dropped within only a small range.
In accordance with the invention, there is provided a tunable add/drop filter capable of selectively adding or dropping one or more wavelength over a wide range of wavelengths. The tuning range can be made large enough to cover the entire bandwidth of a WDM system. The specific wavelength can be selected after fabrication. Post-fabrication tunability over a wide range is a highly desirable feature. It allows, for example, devices to be adjusted during installation. Control signals are adjusted to select a wavelength and the device is left to operate continuously at the selected wavelength, or the device could be electronically and remotely driven to dynamically switch between different wavelengths (such as in optical add/drop multiplexers) allowing fast reconfigurability of optical networks.
The add/drop filter includes two waveguides coupled by an element containing at least two resonators. It is the coupling element that determines the transfer properties of the add/drop filter such as the wavelength of the transferred electromagnetic field. The wavelength of the coupling element in the invention is tunable over a wide range of wavelengths.
In accordance with one embodiment of the invention there is provided a tunable electromagnetic field frequency filter having an input waveguide which carries a signal including at least one desired frequency, and an output waveguide. A resonator-system is coupled to the input and output waveguides operable for the selective transfer of the at least one desired frequency to the output waveguide. The resonator-system supports at least two system modes, and includes at least three reflectors with at least two different reflectivity spectra. At least one of the reflectivity spectra is tuned such that at least two of the system-modes have substantially the same frequency when the transfer occurs substantially.